A fan is a turbomachine which transfers energy to a gas passing through it so that this gas can be conveyed, for example through a wall, into one or more pipes, or alternatively so as to sweep a space, notably to ensure the uniformity thereof a fan is made up of blades fixed to a hub, the assembly forming an impeller, in which said hub is driven by a motor; fan classes are usually defined according to the type of impeller, these differing from one another in terms of the shape of the blades.
What is meant by an “axial fan” is a fan in which the vanes are of helicoid shape and in which the direction in which the gas flows is essentially axial; these fans are also known as “axial-flow fans”.
Axial fans are notably suited to stirring a gas in high-volume premises or to extracting gases through walls; they may also be mounted inside a duct. An axial fan comprises a case ring which is a static part, fixed directly or by means of intermediate components to a building or to some equipment; the case ring exhibits symmetry of revolution extending along a longitudinal axis. This axis will be referred to hereinafter as the “axis of the fan”. The case ring may be of substantially cylindrical or substantially frustoconical shape; part of the duct may constitute the case ring of an axial fan; the two ends of the case ring, which are substantially perpendicular to the axis of the fan, comprise an opening to allow the passage of the gas. An axial fan also comprises blades, also referred to as vanes, fixed to a hub extending along the axis of the fan. Said hub is driven by a motor. The gas is thus driven essentially along the axis of the fan, inside the case ring.
Usually, the motor of an axial fan is positioned inside the case ring, along the axis of the fan. The gas thus moves in the space lying between the internal wall of the case ring and the external wall of the motor.
This results in a loss of pressure head caused by the motor taking up space inside the case ring.
Moreover, the requirements for securing (making safe) industrial equipment are ever increasing and the risks associated with contact between electrical installations and gases, notably explosive gases, need to be carefully taken into consideration. It is therefore appropriate to use special motors, referred to as explosion proof. This results in a significant on-cost.